Learning Objectives

• Define a molecule and differentiate between molecules of elements and compounds.

• Explain the concept of atomicity and classify molecules accordingly.

• Understand the laws of chemical combination that govern how molecules are formed.

• Learn to write chemical formulas for simple compounds using the criss-cross method.

• Calculate molecular mass and understand its connection to the mole concept.

Key Vocabulary

Molecule

The smallest particle of an element or compound that retains the chemical properties of that specific substance.

Atomicity

The total number of atoms that are present in a single molecule of an element or a compound.

Ion

An electrically charged atom or group of atoms formed by the noticeable loss or gain of electrons.

Chemical Formula

A representation of a compound that shows the elements present and the number of atoms of each element.

Molecular Mass

The sum of the atomic masses of all atoms present in one molecule of a particular substance.

Mole

A specific quantity of a substance containing exactly 6.022 x 10²³ elementary entities of that given substance.

What is a Molecule?

Molecules of an Element

• Molecules of an element are composed of atoms of the very same type.

• These identical atoms are held together by strong chemical bonds to form the molecule.

• An oxygen molecule (O₂) contains two oxygen atoms chemically bonded together.

Molecules of a Compound

• Molecules of a compound are made of atoms from different elements.

• These atoms combine in a specific, fixed proportion to form the molecule.

• A water molecule (H₂O) has two hydrogen atoms and one oxygen atom.

Solved Example 1
Calculate the molecular mass of a water molecule (H₂O), given the atomic mass of Hydrogen is 1 u and Oxygen is 16 u.

Step 1: Analyze and Sketch the Problem

• Goal: Find the molecular mass of a water molecule.

• Knowns: The chemical formula is H₂O. The atomic mass of Hydrogen (H) is 1 u. The atomic mass of Oxygen (O) is 16 u.

• Unknown: The molecular mass of H₂O.

• Formula: The molecular mass is the sum of the atomic masses of all atoms in the molecule.

Solved Example 1
Calculate the molecular mass of a water molecule (H₂O), given the atomic mass of Hydrogen is 1 u and Oxygen is 16 u.

Step 2: Solve for the Unknown

Solved Example 1
Calculate the molecular mass of a water molecule (H₂O), given the atomic mass of Hydrogen is 1 u and Oxygen is 16 u.

Step 3: Evaluate the Answer

• The calculated molecular mass of water is 18 u.

• The calculation correctly sums the masses of two hydrogen atoms and one oxygen atom, as per the chemical formula H₂O.

• The final answer is presented in the correct atomic mass units (u).

Atomicity

Monoatomic Molecules

• These molecules consist of only a single atom of an element.

• They are stable and exist independently under normal conditions.

• Examples include Helium (He) and other noble gases like Neon (Ne).

Diatomic Molecules

• These molecules are composed of two atoms bonded together.

• The two atoms can be of the same or different elements.

• Examples are Hydrogen (H₂) and Oxygen (O₂).

Polyatomic Molecules

• These are molecules containing more than two atoms chemically bonded.

• This category includes triatomic molecules like Ozone (O₃).

• Other examples are Phosphorus (P₄) and Sulphur (S₈).

Solved Example 2
The chemical formula for glucose is C₆H₁₂O₆. What is its atomicity, and how many total atoms are present in 5 molecules of glucose?

Step 1: Analyze and Sketch the Problem

• Goal: Find the atomicity of one glucose molecule and the total number of atoms in five molecules of glucose.

• Knowns: The chemical formula is C₆H₁₂O₆. The number of molecules is 5.

• Unknown: The atomicity of glucose and the total number of atoms.

Solved Example 2
The chemical formula for glucose is C₆H₁₂O₆. What is its atomicity, and how many total atoms are present in 5 molecules of glucose?

Step 2: Solve for the Unknown

• First, find the atomicity of C₆H₁₂O₆ by adding the atoms of each element: 6 (Carbon) + 12 (Hydrogen) + 6 (Oxygen) = 24 atoms.

• Next, calculate the total atoms in 5 molecules by multiplying the atomicity by the number of molecules: 24 atoms/molecule × 5 molecules = 120 atoms.

Solved Example 2
The chemical formula for glucose is C₆H₁₂O₆. What is its atomicity, and how many total atoms are present in 5 molecules of glucose?

Step 3: Evaluate the Answer

What are Ions?

Cation

• A cation is an ion that carries a positive electrical charge.

• It is formed when a neutral atom loses one or more electrons.

• For instance, a sodium atom forms a sodium ion, Na⁺, by losing an electron.

Anion

• An anion is an ion that carries a negative electrical charge.

• It is formed when a neutral atom gains one or more electrons.

• For example, a chlorine atom forms a chloride ion, Cl^-, by gaining an electron.

Solved Example 3
A compound is formed from magnesium (Mg²⁺) and chloride (Cl^-) ions. Determine its chemical formula and calculate its formula unit mass. (Atomic mass: Mg = 24 u, Cl = 35.5 u)

Step 1: Analyze and Sketch the Problem

• Goal: Determine the chemical formula and formula unit mass of the compound formed by Magnesium and Chloride ions.

• Knowns: Cation is Magnesium (Mg²⁺), Anion is Chloride (Cl^-). Atomic mass of Mg = 24 u, Atomic mass of Cl = 35.5 u.

• Unknowns: Chemical Formula = ?, Formula Unit Mass = ?

• Formulas: Criss-cross method for the formula; Sum of atomic masses for the formula unit mass.

Solved Example 3
A compound is formed from magnesium (Mg²⁺) and chloride (Cl^-) ions. Determine its chemical formula and calculate its formula unit mass. (Atomic mass: Mg = 24 u, Cl = 35.5 u)

Step 2: Solve for the Unknown

Solved Example 3
A compound is formed from magnesium (Mg²⁺) and chloride (Cl^-) ions. Determine its chemical formula and calculate its formula unit mass. (Atomic mass: Mg = 24 u, Cl = 35.5 u)

Step 3: Evaluate the Answer

• The chemical formula MgCl₂ is correct because the charges are balanced: (+2) + 2(-1) = 0.

• The formula unit mass of 95 u correctly represents the total mass of one magnesium atom and two chlorine atoms.

Laws of Chemical Combination

Law of Conservation of Mass

• This law states that mass can neither be created nor destroyed during any chemical reaction.

• Therefore, the total mass of the reactants must equal the total mass of the products formed.

• A balance scale can show that the mass of reactants equals the mass of the products.

Law of Constant Proportions

• Elements in a pure chemical substance are always present in a definite proportion by their mass.

• For example, in pure water H₂O, the mass ratio of hydrogen to oxygen is always 1:8.

• This fixed mass ratio holds true for any sample of a pure compound, regardless of its origin.

Solved Example 4
According to the Law of Constant Proportions, hydrogen and oxygen combine in a 1:8 mass ratio to form water. What mass of oxygen gas is required to react completely with 3 g of hydrogen gas?

Step 1: Analyze and Sketch the Problem

Solved Example 4
According to the Law of Constant Proportions, hydrogen and oxygen combine in a 1:8 mass ratio to form water. What mass of oxygen gas is required to react completely with 3 g of hydrogen gas?

Step 2: Solve for the Unknown

Solved Example 4
According to the Law of Constant Proportions, hydrogen and oxygen combine in a 1:8 mass ratio to form water. What mass of oxygen gas is required to react completely with 3 g of hydrogen gas?

Step 3: Evaluate the Answer

Dalton's Atomic Theory

• His atomic theory is the foundation of modern chemistry and describes atoms.

• Atoms are indivisible and are not created or destroyed in chemical reactions.

• Atoms combine in whole-number ratios to form compounds like H₂O.

• A given compound always has the same relative number and types of atoms.

How to Write Chemical Formulas

• A chemical formula shows the elements and number of atoms in a compound.

• Write ion symbols and charges, then criss-cross the charge values as subscripts.

• For aluminium chloride, Al³⁺ and Cl^-1 cross over to form the compound AlCl₃.

• For Mg²⁺ and (OH)^-1, use brackets to write the formula Mg(OH)₂.

Solved Example 6
Calculate the molecular mass of Calcium Phosphate, Ca₃(PO₄)₂. The atomic mass of Calcium (Ca) is 40u, Phosphorus (P) is 31u, and Oxygen (O) is 16u.

Step 1: Analyze and Sketch the Problem

• Goal: Find the molecular mass of Calcium Phosphate.

• Knowns: The formula is Ca₃(PO₄)₂. Atomic masses are Ca=40u, P=31u, O=16u.

• Unknown: The total molecular mass of the compound.

• Formula: Molecular Mass = Sum of the atomic masses of all atoms in the formula.

Solved Example 6
Calculate the molecular mass of Calcium Phosphate, Ca₃(PO₄)₂. The atomic mass of Calcium (Ca) is 40u, Phosphorus (P) is 31u, and Oxygen (O) is 16u.

Step 2: Solve for the Unknown

• First, determine the number of atoms of each element. In Ca₃(PO₄)₂, there are 3 Ca atoms, 2 P atoms (2*1), and 8 O atoms (2*4).

• Mass of Calcium = 3 × 40u = 120u.

• Mass of Phosphorus = 2 × 31u = 62u.

• Mass of Oxygen = 8 × 16u = 128u.

• Total Molecular Mass = 120u + 62u + 128u = 310u.

Solved Example 6
Calculate the molecular mass of Calcium Phosphate, Ca₃(PO₄)₂. The atomic mass of Calcium (Ca) is 40u, Phosphorus (P) is 31u, and Oxygen (O) is 16u.

Step 3: Evaluate the Answer

• The calculation correctly sums the masses of all constituent atoms based on the chemical formula. The use of parentheses in the formula indicates that the subscript '2' applies to both P and O within the phosphate ion.

• The final answer of 310u is the correct molecular mass for Calcium Phosphate.

Molecular Mass and Formula Unit Mass

Molecular Mass

• ​It is the sum of the atomic masses of all atoms in one molecule.

• ​​This is used for covalently bonded substances that exist as separate molecules.

• ​The molecular mass of water (H₂O) is (2 × 1.0 u) + 16.0 u = 18.0 u.

Formula Unit Mass

• ​It is the sum of atomic masses of all atoms in a formula unit.

• ​​This is used for substances where the constituent particles are ions, like ionic compounds.

• ​The formula unit mass of sodium chloride (NaCl) is 23.0 u + 35.5 u = 58.5 u.

The Mole Concept

• The mole is a unit for measuring the amount of a substance.

• ​It connects a substance's mass to the number of particles it contains.

• One mole contains 6.022 x 10²³ particles, called Avogadro's constant.

• One mole of water (H₂O) has a mass of 18 grams.

Solved Example 8
Calculate the number of molecules present in a 22 g sample of carbon dioxide (CO₂). The atomic mass of Carbon is 12 u and Oxygen is 16 u.

Step 1: Analyze and Sketch the Problem

• Goal: Find the number of CO₂ molecules in 22 g.

• Knowns: Mass of CO₂ = 22 g; Atomic mass of C = 12 u; Atomic mass of O = 16 u.

• Constants: Avogadro's number = 6.022 x 10²³ particles/mole.

• Plan: First, find the molecular mass of CO₂. Then, use it to convert the given mass to moles. Finally, convert moles to the number of molecules.

Solved Example 8
Calculate the number of molecules present in a 22 g sample of carbon dioxide (CO₂). The atomic mass of Carbon is 12 u and Oxygen is 16 u.

Step 2: Solve for the Unknown

Solved Example 8
Calculate the number of molecules present in a 22 g sample of carbon dioxide (CO₂). The atomic mass of Carbon is 12 u and Oxygen is 16 u.

Step 3: Evaluate the Answer

• The given mass (22 g) is half the gram molecular mass (44 g), so the sample should contain half a mole of molecules.

• Half of Avogadro's number is approximately 3.011 × 10²³, which matches our calculated answer.

Common Misconceptions in Chemistry

Summary

• Atoms combine via chemical bonds to form molecules.

• The number of atoms in a molecule is known as its atomicity.

• A chemical formula represents the composition of a substance.

• The mole concept relates mass to the number of particles.